1. Field of the Invention
This invention relates to endless belts for conveyors and, more particularly, to thermoplastic endless belts driven by sprockets.
2. Description of the Related Art
Low tension, direct drive conveyor belts are typically used in situations where hygiene and cleanliness are critically important. For example, in food processing plants such as those that process meat products for human consumption, low tension, direct drive belt conveyors are used to transport items. Sanitation is critically important and, therefore, the endless belts used in such conveyors are conventionally made of materials that can be hygienically cleaned, such as thermoplastics or stainless steel.
Known belts are typically formed of interlocking links having teeth that are adapted to engage drive sprockets. One of the problems with such belts is that food particles can become lodged in the joints of the interconnecting links. Consequently, cleaning the belts can be difficult and may require removing the belt from the conveyor system for special cleaning operations.
One solution to this problem is the use of flexible thermoplastic belts without interlocking links such as that disclosed in U.S. Pat. No. 5,697,491. Such belts having a smooth continuous surface (sometimes called “homogeneous belts”) are driven by V-guides wherein a radial groove in a drive pulley engages a longitudinal rib on the underside of the belt. One of the problems with such belts is that grease and oil from the food items can migrate to the groove or to the rib, which causes a loss of friction between the pulley and the belt. Consequently the driving force becomes unstable and unreliable. Moreover, such belts are under tension to ensure that the pulley imparts enough driving force. This tensioning raises other issues beyond slippage due to oils and contaminants. A thermoplastic belt under tension will stretch, which may require adjustment of the tension from time to time. In addition, there are additional costs associated with ensuring that the conveyor frame be sufficiently strong enough to handle the normal stresses of the pretensioned belt plus additional stresses caused by loading the belt.
It is known to provide a drive sprocket or drum with transverse grooves that are complementary in shape to teeth on a flexible conveyor belt, as shown for example in U.S. Pat. No. 4,170,281. However, the belt is formed from interlocking links and the belt is still under tension. The problems associated with interlocking links and pretensioning remain.
Another solution is disclosed in U.S. Pat. No. 5,911,307 where a timing belt is added to a homogeneous belt to engage a drive sprocket. As a result, reliance upon friction for motion is minimized, and the belt need not be under tension. There are some remaining problems, however. Assembling a timing belt to a homogeneous belt is costly and the bonding or adhering process is critical. Failure of the bond increases the risk of contamination and total belt failure.